New designs of footwear, and particularly footwear intended for sports or other active wear, have provided improvements in the support, cushioning and stability in an effort to reduce injuries to the feet, ankles and knees. One aspect of active wear footwear design which has been overlooked, however, is the configuration and surface characteristics of the insole or sockliner of the article of footwear as they relate to the ability of the insole to maintain the foot and sock insulated from the sole of the footwear, and the extent which the foot and sock are permitted to move within the article of footwear.
One problem with many insoles for active wear footwear involves a failure to control the motion of the sock of the wearer relative to the insole and/or the motion of the foot of the wearer with respect to the sock. This affects both the comfort and performance of the shoe. For example, certain activities such as the play of tennis on clay courts and soccer on grass result in substantial movement of the shoe with respect to the playing surface. In these types of activities, it is desirable to limit the movement of the foot and sock with respect to the insole of the article of footwear for added comfort and to optimize the performance of the footwear. On the other hand, comfort and performance of the article of footwear dictate that the foot and sock be permitted more movement within footwear intended for use in activities such as basketball, racquetball and aerobics which are typically played on a lacquered hardwood floor wherein limited movement of the article of footwear relative to the playing surface is permitted and therefore relatively high shear forces are transmitted from the footwear to the foot.
Prior art insoles can generally be divided into two categories, both of which fail to take into account the movement of the foot and/or sock within the article of footwear and the type of surface on which the footwear is utilized. In some designs, the top surface of the insole is formed of a tacky or sticky material, or a material which becomes relatively tacky when exposed to moisture from the foot. Insoles of this type exhibit a higher coefficient of friction than the coefficient of friction of the skin of the foot. As a result, the magnitude of the frictional engagement between the sock and insole is greater than the magnitude of the frictional engagement between the foot and sock. Articles of footwear provided with this type of insole have been found to create blisters on the foot during use because the foot is allowed to move within the sock in response to the application of a shear force, i.e., a front-to-rear and/or a side-to-side foot motion, while the sock is held in an essentially fixed position atop the insole. The rubbing motion of the foot within the sock can create severe blistering and discomfort, particularly in activities such as basketball and the like played on hardwood floors which permit limited motion of the shoe therealong.
Another general category of insole designs comprises a rubber or foam bottom layer which is covered by an overlayer of cloth or leather having a relatively slippery or slick surface with a much lower coefficient of friction compared to that of the skin. Insoles of this type help avoid the blistering problem because the foot and sock can move as a unit relative to the slippery top layer of the insole, instead of the foot moving within the sock. But the problem with these insoles is that movement of the sock and foot of the wearer is often completely unrestricted, and the toes are permitted to violently slide into the toe portion of the article of footwear causing bruising or even fractures. In addition, undue movement of the foot and sock gives the wearer a feeling of lack of control of the footwear, particularly in activities where the footwear readily slides along the playing surface.
These problems have been addressed in U.S. Pat. No. 4,893,418, owned by the assignee of this invention. The insole disclosed in such patent comprises a bottom layer formed of a cushioning material such as rubber or foamed plastic having an upper surface, and a lower surface adapted to overlie the sole of an article of footwear such as a shoe. The insole also includes a top layer formed of a non-absorbent, thermally non-conductive thermoplastic material having a plurality of apertures which define intersecting columns and rows of thermoplastic strands or wall sections. The top layer is at least partially embedded in the bottom, cushioning layer so that a portion of the top layer extends beneath the upper surface of the bottom layer and the cushioning material forming the bottom layer at least partially enters the apertures in the top layer.
One advantage of the insole of U.S. Pat. No. 4,893,418 is that the frictional characteristics of the upper surface of the top layer are variable to control the movement of the foot and sock with respect to the insole, depending upon the type of activity and playing surface for which a particular article of footwear is designed. For example, in order to reduce or prevent blistering of the foot, the coefficient of friction of the apertured top layer is chosen such that the magnitude of the frictional engagement between the sock and such top layer is greater than the magnitude of the frictional engagement between the foot and sock during a given activity. As a result, the foot and sock move together as a unit with respect to the top layer of the insole in response to the application of a shear force to the foot instead of the foot sliding within the sock. Because the foot does not move with respect to the sock, the foot is substantially protected from the development of blisters and other problems created by sliding motion within an article of footwear.
While the sockliner or insole disclosed in U.S. Pat. No. 4,893,418 provides a number of advantages over other insoles, it has been found that some potential problems can arise with the use of such insole in articles of footwear, particularly those intended for certain types of vigorous activities such as basketball, racquetball, etc. As mentioned above, the apertures in the top layer of the insole form a matrix of interconnected wall sections, such as squares, triangles or the like. These wall sections are on the order of about 0.6 millimeters in thickness and about 0.5 millimeters in width. The thermoplastic material utilized to form this relatively thin apertured top layer exhibits good strength in compression, but is comparatively weak in shear. As a result, front-to-back and/or side-to-side motion of the sock along the apertured top layer has a tendency to stretch, pull or otherwise move the wall sections of the top layer relative to one another. The resilient cushioning material within which the apertured top layer is embedded offers substantially no resistance to the application of such shear forces and thus readily permits such relative motion of the wall sections. The apertured top layer is therefore subject to tearing or ripping of its wall sections, and the cushioning material beneath can become worn and break down as the apertured top layer moves therealong.
Another potential problem in the use of the insole disclosed in U.S. Pat. No. 4,893,418 in certain types of applications is that the apertured top layer can become delaminated from the cushioning layer. As disclosed in such patent, the apertured top layer and cushioning layer are interconnected by introducing the top layer onto the cushioning layer when it is in a "foamed" state, i.e., wherein the material has the consistency of whipped cream or the like before it is cured to a solid sheet. Alternatively, the apertured top layer can be molded to cushioning materials such as polyurethane which is liquid when initially combined with the top layer and thereafter cures to form a solid layer. In either case, the only connection between the apertured top layer and cushioning layer is the extent of surface contact between the cushioning material and the bottom and sides of the wall sections of the apertured top layer. This is a relatively small surface area. Additionally, the wall sections are made relatively smooth to provide comfort when contacted by the foot or sock of the wearer, which further increases the difficulty of obtaining a secure bond between the top layer and cushioning layer sufficient to avoid delamination.